(1) Field of the Invention
The invention relates to a voltage controlled oscillator, and especially relates to a dual positive-feedbacks voltage controlled oscillator for improving the efficiency of the voltage controlled oscillator in the microwave band at the performance of the low supply voltage.
(2) Description of the Prior Art
With the wireless communication technology flourishing, the quality determinant of the receiver transmitter applied in the wireless communication system is decided by the quality of variable frequency signal generated from the voltage controlled oscillator (VCO). Otherwise, the voltage controlled oscillator is usually applied in signal generator
The VCO is an electronic oscillator designed to be controlled in the oscillation frequency by a voltage input. The frequency of oscillation is varied by the applied direct-current(DC) voltage, while the modulating signals is also able to be fed into the VCO to cause the frequency modulation.
However, the conventional voltage controlled oscillator designed at the standard voltage uses a negative resistance generating circuit for offsetting the parasitic resistance of the inductor-capacitor (also commonly LC) tank, and the negative resistance generating circuit is such as n-type metal-oxide-semiconductor field effect transistors (also commonly NMOS or p-type metal-oxide-semiconductor field effect transistors (also commonly PMOS) cross-coupled pair circuit, single-ended oscillation circuit or Colpitts circuit; otherwise, the conventional voltage controlled oscillator uses a dual negative resistance generating circuit such as complementary cross coupled pair circuit with NMOS-PMOS structure, so the circuit type of the dual negative resistance generating circuit is not in accordance with the design of the low voltage. Therefore, said methods need a large direct current for offsetting in the design of the low voltage, so the power consumption is raised. Otherwise, due to the low voltage, the signal to noise ratio (SNR) of said voltage controlled oscillators is decreased, and the performance of the phase noise is in attenuation.
Consequently, how to decrease the phase noise and the power of the VCO at the operation of the low voltage is an important issue.